An electric power steering apparatus (EPS) serves as an apparatus where a driving section is equipped with a motor. The electric power steering apparatus provides a steering mechanism of a vehicle with a steering assist force (an assist force) by means of a rotational force of the motor, and applies a motor driving force controlled with an electric power supplied from an inverter as the steering assist force to a steering shaft or a rack shaft by means of a transmission mechanism such as gears. In order to accurately generate the assist torque of the steering assist force, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of a pulse width modulation (PWM)-control. A brushless motor that is superior in maintainability is commonly used as the motor.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, a handle shaft) 2 connected to a steering wheel (a handle) 1 is connected to steered wheels 8L and 8R through reduction gears 3 in a reducing section, universal joints 4a and 4b, a rack and pinion mechanism 5, tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angel θ, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (a steering assist) command based on the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for EPS based on a voltage control command value Vref obtained by performing compensation and so on with respect to the current command value.
As well, the steering angle sensor 14 is not indispensable and may not be provided, and it is possible to obtain the steering angle from a rotational position sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 to send/receive various information and signals on the vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vs from the CAN 40. Further, it is also possible to connect a non-CAN 41 to the control unit 30 sending/receiving a communication, analog/digital signals, a radio wave or the like except the CAN 40 to the control unit 30.
The control unit 30 mainly comprises a CPU (Central Processing Unit) (including an MCU (Micro Controller Unit), an MPU (Micro Processor Unit) and so on), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31 calculating a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a current supplied to the motor 20 based on the steering torque Th and the vehicle speed Vel and by means of an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm limited the maximum current is inputted into a subtracting section 32B, and a deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im being fed back is calculated. The deviation I is inputted into a proportional-integral (PI)-control section 35 for improving a characteristic of the steering operation. The voltage control command value Vref whose characteristic is improved by the PI-control section 35 is inputted into a PWM-control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37 serving as a driving section. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. The inverter 37 uses field effect transistors (FETs) as driving elements and is comprised of a bridge circuit of FETs.
A compensation signal CM from a compensation signal generating section 34 is added to the adding section 32A, and a characteristic compensation of the steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation signal generating section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 in an adding section 344, further adds the result of addition performed in the adding section 344 with a convergence 341 in an adding section 345, and then outputs the result of addition performed in the adding section 345 as the compensation signal CM.
Such a motor used in the electric power steering apparatus is generally a brushless synchronous type motor, a rotor of the synchronous type motor is provided with permanent magnets on a surface or inside of the rotor and is rotated by an interaction between the permanent magnets and a rotating magnetic field generated by a coil wound on slots of a stator side. An axis which a magnetic flux formed by the permanent magnets penetrates the rotor in a diameter direction is named a d-axis, and an axis which a magnetic flux formed by a stator coil of the stator penetrates the rotor in a diameter direction is named a q-axis. There is also a motor effectively using a reluctance torque (a reaction torque) by increasing an inductance Lq of a vertical axis (the q-axis) greater than an inductance Ld of the d-axis, while the magnetic flux of a horizontal axis (the d-axis) direction due to an armature current passes smoothly in a rotor iron core by arranging salient pole portions between the permanent magnetics mounted on the rotor circumference.
An overall structure of a general three-phase synchronous motor 200 will be explained with reference to FIG. 3, the three-phase synchronous motor 200 comprises a stator on which a coil 211 is wound, a rotor 220, and a case 230 for containing them. A circumference surface of the rotor 220 is provided with permanent magnets 221, and a rotational shaft 222 of the shaft center is rotatably and pivotally supported by bearings 231 and 232 mounted in the case 230.
An output torque Ts of such the synchronous type motor using the permanent magnets is obtained by the below Equation 1.Ts=Tm+Tr  [Equation 1]                where, Tm is a torque due to a magnetic flux ϕm of the permanent magnet, and Tr is a reluctance torque.The reluctance torque Tr is obtained by the below Equation 2.Tr=P(Lq−Ld)·Iq·Id  [Equation 2]        where, P is number of pole pairs, Lq is a q-axis inductance, Ld is a d-axis inductance, and Iq and Id are respective axis components of the armature current.From the Equation 2, in general, it is understood that it is capable of increasing the reluctance torque Tr when the q-axis inductance Lq is great and the d-axis inductance Ld is small. As well, the torque Tm due to the permanent magnets is formed by the following Equation 3.Tm=ϕm·Iq  [Equation 3]        
However, the general synchronous type motor having the salient pole portions stays that a little investigation is added with regard to the shape, the arrangement and so on of the salient pole, and it would be difficult to sufficiently apply the investigation on a rotor structure utilizing the reluctance torque Tr at the maximum. Therefore, room to increase the output torque Ts of the motor and to miniaturize the motor shape at the same torque is remained by effectively using the reluctance torque Tr.
A synchronous type motor for solving such the problem is, for example, described in Japanese Unexamined Patent Publication No. 07-39031 A (Patent Document 1).
With regard to the synchronous type motor described in Patent Document 1, as shown in FIG. 4, the rotor 220 comprises four salient pole portions 223 to 226 at orthogonal positions, and respective centers of the salient pole portions 223 to 226 are provide with slits 223A to 226A. Further, inner circumference-side end portions of the slits 223A to 226A are respectively extended to a circumference direction of the rotor 220 and form circumference direction slits 223B to 226B. An outer circumference surface of the rotor 220, which is intermediate positions of the salient pole portions 223 to 226, is provided with the permanent magnets 221 throughout a shaft direction. As well, any the salient pole portions 223 to 226 are made from the magnetic material having a high magnetic permeability.
The slit is air gap for the magnetic flux, and the magnetic permeability is low. Since the magnetic flux attempts to form a magnetic path so as to avoid the slits 223A to 226A being the air gap, a magnetic path Md being formed by the permanent magnets 221 passes the further inner circumference side than the circumference direction slits 223B to 226B and passes from respective teeth 227 against the permanent magnets 221 to a yoke site of the stator 210. Since the salient pole portions 223 to 226 present between the two permanent magnets 211 are divided at a diameter direction by the slits 223A to 226A serving as the air gap, a small loop-shape magnetic path is not formed in the inner site of the respective salient pole portions 223 to 226. Therefore, the d-axis inductance Ld becomes much smaller. On the other hand, a magnet path Mq which is formed in order to pass the salient pole portions 223 to 226 due to the conducting toward the stator coil passes the outer circumference side of the circumference direction slits 223B to 226B and further passes from slots 228 opposite to the respective salient pole portions 223 to 226 to the yoke site. The q-axis inductance Lq is substantially larger than the d-axis inductance Ld.
As a result, since the reluctance torque Tr becomes greater, the distance (Lq−Ld) becomes greater based on the above Equation 2, and the output torque Ts of the synchronous type motor increases greater than the torque having the conventional simple salient pole portions.